1. Field of the Invention
The present invention relates to a method for increasing and varying the production capacity of sulfuric acid processes and, in particular, increasing the production of concentrated sulfuric acid solutions by producing more sulfur trioxide without an increase of sulfur dioxide emissions.
2. Description of the Prior Art
Sulfuric acid is the highest volume chemical manufactured in the world. Its production volume has been historically used to measure the industrial development of nations and societies. Current worldwide capacity is estimated at about 570,000 tons per day. About 30%, or 170,000 tons per day, of the world capacity is located in the United States.
Most of the sulfuric acid produced is consumed to produce phosphoric acid in integrated fertilizer complexes. Typically, several sulfuric acid plants will be co-located in such industrial complexes. For example, several large United States fertilizer complexes host multiple sulfuric acid plants which generate over 10,000 tons of sulfuric acid per day.
The principal raw materials used to make sulfuric acid are, first, an oxidizable sulfur-containing material, such as elemental sulfur itself, iron pyrite or other sulfide ores, and hydrogen sulfide, and second, a decomposable sulfate such as calcium sulfate or spent (contaminated and diluted) sulfuric acid. In addition, an oxygen-containing oxidizing gas such as air or oxygen and also water are necessary for the processing. In most types of plants, the first stage of the process has the objective of producing a reasonably continuous, essentially contaminant-free gas stream containing essentially sulfur dioxide, oxygen, and nitrogen, by oxidation of the sulfur-containing feed material in a kiln or other suitable thermal combustion zone. When spent sulfuric acid is used as a raw material for producing the desired sulfur dioxide and/or trioxide, it is injected as a liquid spray into the combustion zone and there mixed with a carbonaceous material such as fuel to provide the heat necessary for evaporation of the water content of the spent sulfuric acid and for decomposition of H.sub.2 SO.sub.4 into H.sub.2 O, SO.sub.3, SO.sub.2 and O.sub.2 when the mixture is burnt.
A vanadium/potassium sulfate catalyst supported on a diatomaceous earth carrier is typically used in the next stage, a catalytic oxidation stage, to convert sulfur dioxide and oxygen in the process-gas stream to sulfur trioxide in a heterogeneous-type reaction (gas phase bulk reaction with adsorbed solid and molten salt phase intermediate steps). However, other catalysts also well-known in the art are likewise potentially usable in this catalytic oxidation reaction although only the vanadium-type catalysts are used commercially. If the initial oxygen concentration of the process gas is low, additional air or oxygen is added prior to or during catalytic oxidation to ensure that them is an excess over stoichiometric needs for conversion of gaseous sulfur dioxide to gaseous sulfur trioxide. After conversion of the sulfur dioxide to sulfur trioxide, the sulfur trioxide is reacted with water to form sulfuric acid in an SO.sub.3 absorption zone, which is typically a tower packed with 3" ceramic saddles or less commonly a Venturi scrubber. In either one, the absorption media is a strong sulfuric acid solution (96-99.8% H.sub.2 SO.sub.4).
While reaction of SO.sub.3 with the water portion of the concentrated sulfuric acid is rapid and virtually complete, SO.sub.2 is removed from the gas phase less well and is represented by the equilibrium reaction forming sulfurous acid (H.sub.2 SO.sub.3) in concentrated sulfuric acid media as follows: EQU SO.sub.2 +H.sub.2 O.revreaction.H.sub.2 SO.sub.3
Of course, the extent of reaction is dependent upon the reaction medium, and in particular, the concentration of sulfuric acid in the reaction medium, as well as the temperature of the reaction medium. In practice, those skilled in the art may refer to solubility charts, tables, or diagrams to determine the amount of SO.sub.2 that dissolves in a given concentrated sulfuric acid solution at a given temperature. Since at least some of the sulfur dioxide does not dissolve, those skilled in the art will also recognize that the gas leaving the absorption tower therefore primarily contains SO.sub.2 as the active sulfur compound.
In most countries in the world, sulfuric acid plants are nowadays limited by the amount of sulfur dioxide that they are allowed to emit to the atmosphere. The U.S. Environmental Protection Agency currently limits sulfur dioxide emissions to four pounds per short ton (2 kg per metric ton) of sulfuric acid H.sub.2 SO.sub.4 produced. This is equivalent to a minimum of 99.7% sulfur dioxide conversion to sulfur trioxide in the catalytic conversion step. Accordingly, this condition represents the limit of maximum practical plant capacity, and most plants operate at this limit or as close as possible to it, because this is the most economical, permissible operating condition.
It is well-known to sulfuric acid plant engineers and designers how the capacity/conversion relationship will be observed in each sulfuric acid plant. This relationship will depend on the amount and condition of the catalyst beds in each plant and on the heat exchange capacity of the ancillary equipment, i.e., the equipment that provides temperature control to the catalytic beds. The exact response will vary from plant to plant depending on catalyst loading (ratio of amount of catalyst to the amount of sulfur dioxide required to be converted) and catalyst condition.
A typical plant response to burning more sulfur is an increase in the sulfur dioxide emissions from the stack. This is because an increase in the amount of sulfur dioxide to be oxidized catalytically results in a shift of the equilibrium in the reactor or converter such that the efficiency of the catalytic reaction from sulfur dioxide to sulfur trioxide is reduced. As a result of the shift in the equilibrium, more unconverted sulfur dioxide remains in the effluent. Thus, when a sulfuric acid plant is pushed to exceed the limit of its capacity by burning more sulfur, the sulfur dioxide emissions through the stack will eventually exceed the 4 lb/ton legal limit. As a result, the plant must reduce its rate or risk legal action.
In cases of higher than desirable sulfur dioxide emissions, the prescribed emission requirements are sometimes met by the use of tail-gas scrubbers added for this purpose, especially in conjunction with low-conversion, single-stage SO.sub.3 absorption plants, and most commonly in existing plants rather than in new plants. The gas treated in such SO.sub.2 tail-gas scrubbers subsequent to SO.sub.3 absorption does not contain any measurable amount of sulfur trioxide under normal conditions.
A number of SO.sub.2 tail-gas scrubbing processes are available. These known tail-gas scrubbing techniques variously depend on ammonia, sodium hydroxide or hydrogen peroxide as the scrubbing liquid, but each suffers from certain disadvantages. For all such tail-gas scrubbing techniques, a separate scrubbing tower to convert unwanted sulfur dioxide emissions must be installed at the tail end of the plant. In addition, when a base such as ammonia is used, a side stream of ammonium sulfate is produced, requiring the marketing of a chemical material which may or may not be easily sold. Further, the reaction of ammonium salt solutions with sulfur dioxide results in submicron aerosol fumes which require sophisticated and expensive mist eliminators for efficient emission control. Scrubbing with sodium hydroxide in a separate tower accomplishes substantially the same result as scrubbing with ammonia. However, the by-product sodium sulfate is less salable and less usable than is ammonium sulfate within the production and marketing confines of a fertilizer manufacturer.
Oxidation with peroxide compounds to eliminate unwanted sulfur dioxide emissions has been described in the prior art, e.g., U.S. Pat. No. 3,917,798. This prior art describes the removal of sulfur dioxide from sulfur dioxide-containing combustion gases by scrubbing them with sulfuric acid solutions and a peroxide compound, subsequent to a previous removal of sulfur trioxide therefrom by absorption or otherwise. The peroxide-based scrubbing solutions disclosed in U.S. Pat. No. 3,917,798 vary in concentration between 0.01% and 25% H.sub.2 O.sub.2 and between 30 and 60% sulfuric acid, but are thereafter further concentrated up to 90% by heat and evaporation, whereby residual H.sub.2 O.sub.2 is decomposed. In other words, these prior teachings only deal with sulfur dioxide abatement, and are not concerned with the sulfuric acid production capacity of sulfuric acid plants, much less with any debottlenecking of their basic plant design. According to such prior art, the SO.sub.2 scrubbing is performed in a terminal scrubbing tower, essentially independent of the sulfuric acid production plant proper. No free sulfur trioxide is present in the gas phase in this stage, as it was substantially completely removed in the SO.sub.3 absorption stage of the plant proper.
In each of the prior SO.sub.2 scrubbing techniques described above, expensive equipment which takes up valuable space is necessary and, in the case of scrubbing with a base, produce a usable but generally unwanted by-product. Besides these disadvantages, the additional scrubber equipment necessary causes a significant, additional pressure drop to the overall gas system and thus a decrease in the gas handling capacity of the system.
The present invention makes it possible to increase and readily vary sulfuric acid production capacity in new or existing plants without requiting installation of separate SO.sub.2 scrubbing equipment and without exceeding sulfur dioxide emissions.